Turnbuckle assemblies for establishing an adjustable axial connection between two coaxial rod members are well known in the art. Such assemblies typically include an internally threaded tubular sleeve. Each end of the sleeve has an internally threaded portion that receives an associated externally threaded rod member. One sleeve end and its associated rod have left-hand threads. The other sleeve end and its associated rod have right-hand threads. As the sleeve is rotated, the rods are both screwed into or out of the sleeve depending on the direction of rotation of the sleeve. After the desired axial spacing between the two rods is achieved, a locking mechanism is used to lock the turnbuckle in place. Fasteners are applied to each end of the sleeve to force the internal sleeve threads tightly against the external rod threads and thereby hold the turnbuckle in place relative to the rods. Such fasteners include ring clamps tightened around the outside of the sleeve and jam nuts tightened axially against the ends of the sleeve.